The invention relates to techniques for preventing chemical vapor deposition (CVD) reaction chamber windows from becoming clouded, and more particularly to a technique utilizing narrow light pipes to introduce light into the reactant gas.
Chemical vapor deposition processes are well-known and are used extensively in the manufacture of semiconductor integrated circuits. To avoid subjecting semiconductor wafers in CVD processes to high temperatures that may cause various problems such as lateral diffusion of dopants in highly doped region of the integrated circuit, generation of material defects, and generation of undesired stresses in the semiconductor material, various photo-CVD or photo-assisted CVD deposition processes have been developed wherein the substrate is irradiated with ultraviolet (UV) light while it is being heated, allowing significantly lower temperatures to be used.
Photo-assisted CVD processes have not been widely used to date because no practical technique has been developed for preventing clouding of light windows through which the UV light is introduced into the reaction chamber. The chemical reactants are deposited not only on the semiconductor wafers, but also on the walls of the reactant chamber and the light window thereof.
The deposition of the material on the window clouds it, reducing the amount of UV light entering from one process to the next, and also producing unacceptable non-uniformity in the deposition of oxide layers, nitride layers, and the like on the semiconductor wafers. This results in reduced integrated circuit yields.
Various prior techniques have been used to reduce deposition of material from reactant gases on the window through which UV light is introduced. U.S. Pat. Nos. 4,435,445, 4,556,584, 4,654,226, 4,715,318, 4,778,693, 4,811,684, 4,816,294, 5,005,519 are generally indicative of the state-of-the-art.
There is an unmet need for a practical, economical system for preventing appreciable deposition of material from reactant gases in a photo CVD reaction chamber on the surface of a transparent window thereof through which light is introduced into the reaction chamber.